People make phone calls as law enforcement investigate an explosion at the Dar Al-Farooq Islamic Center in Bloomington, Minn., on Saturday, Aug. 5, 2017. Bloomington police Chief Jeff Potts said Saturday that investigators are trying to determine the cause of the blast. Authorities say the explosion damaged one room but it didn’t hurt anyone. (David Joles/Star Tribune via AP)

BLOOMINGTON, Minn. (AP) — Someone apparently threw a bomb through the window of a suburban Minneapolis mosque on Saturday as people were preparing for morning prayers, damaging the imam’s office but not causing any injuries, authorities said.

Minneapolis police said a preliminary investigation shows a destructive device caused the explosion, “in violation of federal law,” and that the FBI has taken the lead in the investigation. An FBI spokesman didn’t return a phone message Saturday seeking details about the investigation.

The blast happened at around 5 a.m. at the Dar Al-Farooq Islamic Center in Bloomington, south of Minneapolis. Worshippers managed to extinguish the blaze before firefighters arrived, according to a statement from the Muslim American Society of Minnesota.

A witness reported seeing something being thrown at the imam’s office window as about a dozen people gathered for morning prayers, said Asad Zaman, the society’s director. Zaman described it as a firebombing, the Star Tribune reported.

One worshipper saw a pickup truck speeding away afterward, said Mohamed Omar, the center’s executive director. He said the mosque, which primarily serves people from the area’s large Somali community, occasionally receives threatening calls and emails.

“It was 5 a.m.,” Omar told the newspaper. “The whole neighborhood was calm. People were supposed to be sleeping, that’s how peaceful this should be. I was shocked to learn this happened.”

Trevin Miller, who lives across the street, said the explosion woke him up and felt the blast on his “insides.”

Yasir Abdalrahman, a worshipper at the mosque, said the explosion was “unimaginable.”

“We came to this country for the same reason everyone else came here: freedom to worship,” Abdalrahman said. “And that freedom is under threat. Every other American should be insulted by this.”

The mosque serves as a religious center and community organizing platform for Muslim activists and leaders in the area, according to the society. The group is offering a $10,000 reward for information that leads to an arrest or conviction.

A $10,000 reward also is being offered by the Minnesota chapter of the Council on American-Islamic Relations, or CAIR. The group said its national office is urging Islamic centers and mosques to step up security.

The local chapter’s civil rights director, Amir Malik, said the group hopes the reward will help authorities quickly apprehend the perpetrator of the “act of violence.”

“If a bias motive is proven, this attack would represent another in a long list of hate incidents targeting Islamic institutions nationwide in recent months,” Malik said.